Steel Plate Pre-Drilling Requirements

When using self-tapping screws with steel plates, the steel side member must always be pre-drilled prior to the installation of the screws. The pre-drilled holes for ASSY self-tapping screws depend on the diameter of the screw used. The following figure, taken from our Structural Screw Design Guide, summarizes the general pre-drilling requirements in wood and steel members.

Figure 1: Pre-drilling Recommendations for steel and wood members

In both wood and steel members, pre-drilling must be done so that the screw is not loose in the member, with the threads engaging the wood for the full capacity of the connection. 

There are several screws and housing options available for steel-to-wood connections. In general, steel-to-wood connections with ASSY self-tapping screws can be done by: 

  • Having the screw sitting proud on the steel plate 
  • Completely countersinking the screw head 
  • Installing the screw with a washer

The Kombi screws are specifically designed to be used in steel-to-wood connections due to their easy to use hexagonal head shape. The Kombi also includes a shoulder under the head, which provides a snug fit and reduces the initial connection slip. The screw shoulder diameter is as wide as the thread of the screw, so no extra design work is needed.  The placement of the Kombi screw in a steel-to-wood connection is shown in Figure 2 below. Figure 2 also compares the head placement of the washer head screws, the SK and FWH, in steel-to-wood connections. 

Figure 2: Head placement of the (a) Kombi, (b) SK and (c)(d) FWH screws in steel members

The VG CSK or the Ecofast offer reliable alternatives to the Kombi screws for steel-to-wood connections in cases where countersinking the head is desirable. The VG CSK or Ecofast head offers a versatile design that can be countersunk in the steel plate or used with 90-degree and 45-degree washers. Figure 3 below presents some of the countersunk head self-tapping screws installed at 90-degrees into a steel side plate housing options: installed using a 90-degree cup washer (a), it can be countersunk in a milled steel plate for a completely flush finish (b), it can be left sitting proud on the steel place surface.

Figure 3: Head placement of the countersunk head screw (VG CSK or Ecofast)

45-degree washers are typically used with the VG CSK for axially loaded inclined screw connections. Figure 4 shows the countersunk head screw used with the 45-degree washer in a steel-to-wood connection. These washers aid in the installation of the screw by providing the correct angle for installation and provide enough bearing for the screw head while reducing the thickness of the steel plate used. 

Installation with the washer is simple as they only require a punched hole in the steel plate. In contrast, milling the steel plate at an angle to accommodate the screw head may be more difficult. Washers in steel to wood connections offer a more cost-effective solution compared to milling the steel plate to house the screw head. 

Figure 4: Countersunk head screw with 45-degree washer (axially loaded inclined screw)

In Figure 5 the head of the screw is completely countersunk within the milled hole. In steel-to-wood connections to completely countersunk the screw head,  the steel plate must be milled at an angle with the right head dimensions. The screw head dimensions can be found in our Structural Screw Design Guide.

Figure 5: Countersunk head screw milled in steel plate  (axially loaded inclined screw)


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